synchronize access to WmoCodeFlagTables getInstance()

Fixes #709

Author: lesserwhirls

grib/src/main/java/ucar/nc2/grib/grib2/table/WmoCodeFlagTables.java

@@ -47,7 +47,7 @@ public enum TableType {
 
   private static WmoCodeFlagTables instance;
 
-  public static WmoCodeFlagTables getInstance() {
+  public static synchronized WmoCodeFlagTables getInstance() {
     if (instance == null) {
       instance = new WmoCodeFlagTables();
       try {